1. Field of the Invention
This invention relates to anode structures for metal air electrochemical cells.
2. Description of the Prior Art
Electrochemical power sources are devices through which electric energy can be produced by means of electrochemical reactions. These devices include metal air electrochemical cells such as zinc air and aluminum air batteries. Metal air cells generally include an anode, a cathode, a separator to electrically isolate the anode and the cathode, and an electrolyte to conduct ions between the anode fuel material and the cathode. The cathode generally comprises an air diffusion electrode having a catalyzed layer for reducing oxygen. The electrolyte is usually a caustic liquid that is ionic conducting but not electrically conducting. The anode is an oxidizable metal, such as zinc, aluminum, or magnesium.
Metal air electrochemical cells have numerous advantages over traditional hydrogen-based fuel cells. In particular, the supply of energy provided from metal air electrochemical cells is virtually inexhaustible because the fuel, such as zinc, is plentiful and can exist either as the metal or its oxide. Further, solar, hydroelectric, or other forms of energy can be used to electrically convert the metal from its oxide product back to the metallic fuel form with very high energy efficiency. Additionally, certain metal air electrochemical cells may be mechanically recharged, or refueled, by replacing the metal anode fuel. Unlike conventional hydrogen based fuel cells that require refilling, the fuel of metal air electrochemical cells is recoverable by electrically recharging. The fuel of the metal air electrochemical cells may be solid state, therefore, it is safe and easy to handle and store. In contrast to hydrogen based fuel cells, which use methane, natural gas, or liquefied natural gas to provide as source of hydrogen, and emit polluting gases, the metal air electrochemical cells results in zero emission. The metal air fuel cell batteries operate at ambient temperature, whereas hydrogen-oxygen fuel cells typically operate at temperatures in the range of 150° C. to 1000° C. Metal air electrochemical cells are capable of delivering higher output voltages (1-4.5 Volts) than conventional fuel cells (<0.8V).
One of the principle obstacles of metal air electrochemical cells is the inherent volume expansion of the metal, wherein the electrode shape may vary. Electrode shape change generally involves migration of zinc from the certain regions of the electrode to other reasons, and occurs, in part, as the active electrode material dissolves away during battery discharge. Swelling and deformity of zinc electrodes also occur due to the differences in volume of metallic zinc and its oxidation products zinc oxide and zinc hydroxide. Electrode shape distorts as the zinc is redeposited in a dense solid layer, thereby minimizing available active electrode material and preventing electrolyte access to the electrode interior.
Another obstacle relates to refueling of metal air cells. If the clearance between the anode and cathode is not large enough to accommodate the anode expansion, the cathode may be damaged and hence render refueling difficult or impossible. The distance between anode and cathode should be constant. If the distance between the anode and cathode is not constant, the discharging between the anode and cathode will be uneven. This uneven discharging will cause the anode to bend or deform. This bend on the anode is caused by the volume change due to the metal oxidation. When the anode is bent, the anode area which closer to the cathode discharges faster than the rest of the anode. This will increase the deformation. Therefore, the uneven discharging is magnified, and the problem continues until the bending causes cell failure, for example by shorting with the anode. Also, the uneven discharging will reduce the power output of the cell. If the cell is discharged at very high power, the regions of the anode closer to cathode will be passivated and lose functionality.
In order to refuel, the anode and cathode should have certain distance between them to provide the clearance for the refueling action. Conventionally, this clearance is filled with electrolyte and separator. However, this clearance will increase the cell internal resistance. This internal resistance will generate heat during use, which may cause various detriments. The heat consumes power from the cell, will dry out the electrolyte quickly, and speeds up the deterioration of the fuel cell. In order to reduce the internal resistance, the distance between the anode and cathode should be small and even. However, this conventionally sacrifices durability. During the refueling process, if the distance between anode and cathode is not sufficient, the anode may scrape the cathode surface. However, excess clearance, while reducing the likelihood of cathode damage during the refueling, increases the internal resistance. Therefore, conventionally provision of sufficient clearance between the anode and cathode results in increased internal resistance between them.
There remains a need in the art for an improved cell structure, particularly a cell structure that accommodates for anode expansion and allows for easy refuelability.